docs: Add how to use docfx nightly version

[filzrev]

This PR add How to use prerelease version of docfx section to Quick Start page.

By following the instructions in this section.
It can install pre-release version of docfx from GitHub Packages.

[yufeih]

Should we recommend adding GitHub feed URL to NuGet.config?

[filzrev]

Should we recommend adding GitHub feed URL to NuGet.config?

I don't consider it particularly necessary.
GitHub Packages requires PAT token for using public packages.
So It's simpler downloading nupkg by using gh command and REST APIs.

Examples of using GitHub Packages with NuGet.confg

1. [Feature Request] Publish docfx prerelease version NuGet packages #9609 (comment)

2. docfx/.github/workflows/nightly.yml

   Lines 55 to 76 in 5d68cfe

   	- name: Create NuGet.config
   	shell: pwsh
   	run: |
   	@'
   	<?xml version="1.0" encoding="utf-8"?>
   	<configuration>
   	<packageSources>
   	<clear />
   	<add key="github" value="https://nuget.pkg.github.com/dotnet/index.json" />
   	</packageSources>
   	<packageSourceCredentials>
   	<github>
   	<add key="Username" value="%USER%" />
   	<add key="ClearTextPassword" value="%GITHUB_TOKEN%" />
   	</github>
   	</packageSourceCredentials>
   	</configuration>
   	'@ | Out-File NuGet.config -Encoding UTF8
   	- name: Install nightly build package
   	run: |
   	dotnet tool install docfx -g --prerelease

[yufeih]

Ok, people should find how to authenticate against GH packages if they want to use NuGet.config.